A memory device having the following structure has been put to practical use. A control gate is stacked on a floating gate via an intermediate insulating film. The control gate is provided also on the sidewall of the floating gate via the intermediate insulating film. Thus, the capacitive coupling ratio between the control gate and the floating gate is increased.
With the miniaturization of memory devices, the spacing between the adjacent floating gates is narrowed. Then, there is concern that the space between the adjacent floating gates is filled with the intermediate insulating film. If the space between the adjacent floating gates is filled with the intermediate insulating film having high permittivity, interference between the adjacent cells via the intermediate insulating film is more likely to cause a problem such as varying the threshold.
Furthermore, in order to suppress the intercell interference, the following structure has been proposed. The floating gate is configured to have a protruding shape such that its upper portion has a smaller width than its lower portion. Between the adjacent floating gates, an intermediate insulating film is formed only between the upper portions of the floating gates.
The process for forming an intermediate insulating film only between the upper portions of the floating gates having such a protruding shape is very difficult from the viewpoint of dimensional controllability. Thus, a process having high dimensional controllability is required.